Fluxes of ~MeV electrons in near-Earth space over the course of the Van Allen Probes mission, in three different energy ranges from two instruments. The L value on the y-axis scale is the equatorial plane distance of the magnetic field, showing the dynamic outer belt and the stable inner belt. The lower panel shows the daily-averaged Dst index, a measure of geomagnetic activity, reference. Credit: Zhao et al., 2018, Figure 2

The two satellites of NASA’s Van Allen Probes mission have provided a wealth of data suitable for statistical analysis to build up data-based empirical models of the charged particles near Earth. Zhao et al. [2018] provide such a model of the energetic electrons in the radiation belts, but with a new independent variable compared to existing parameterizations: pitch angle. This is the angle at which the charged particles spiral along the magnetic field line. For reference, high pitch angles (near 90 degrees) means that the particles are confined near the magnetic equatorial plane, while a low pitch angles (near zero) means the particles can spiral farther along the field lines, perhaps even all the way to the Earth’s upper atmosphere (where it would collide and be lost). One of their big findings is that there is a significant day-to-night difference in the pitch angle distribution of the highest-energy electrons. Quantifying this dependence as a function of other independent variables will help the community understand the physical processes at work on these particles.

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